Dirt won’t stick to omniphobic material

Now US chemists have created a material antisocial enough to repel liquids of both kinds. They have gone one better than nature, which is not known to have made materials with such properties.

Robert Cohen‘s team at the Massachusetts Institute of Technology, even had to coin a new word to describe their creation – “omniphobic” – literally meaning it hates everything.

Toadstool surface

The material forces away watery and oily liquids into tight droplets due to its surface texture, made up of 300-nanometer-tall “toadstools” with broad silicon dioxide caps and narrow silicon stems.

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All liquids have a surface tension that attempts to pull a drop into a perfect sphere, like those seen in the zero gravity of space. But the strength of that tension varies between liquids.

Water’s very high surface tension, 72 milliNewtons per metre (mN/m) at room temperature, means it easily forms near-spherical drops when placed on a surface. Because of their near-spherical shape, the droplets meet the surface at a high angle – above 150° if the water is sitting on a superhydrophobic surface.

Oils such as pentane have a low surface tension – 15mN/m – so they sag under gravity and tend to form a flat pool rather than a spherical droplet, meeting the surface at a low angle.

All the angles

The shape of the omniphobic toadstools makes it possible for even that weak surface tension to hold a droplet together, allowing liquids like pentane to form a sphere without collapsing, Cohen told New Scientist.

“If you stand on top of one of these [toadstools] and start walking towards the edge, you’ll pass through all angles and eventually you’ll be standing upside down,” he says.

That means even oily liquids can find their ideal angle with the surface and form a meniscus between adjacent toadstools that can support a spherical droplet. The meniscus rests on a layer of air beneath the toadstools’ caps.

Although the toadstools are slightly omniphobic on their own, making it possible to knock droplets of water or oil around them like marbles with ease required adding a surface coating to enhance the effect.

The chemical used – fluorodecyl POSS – is more usually used to make surfaces more hydrophobic. After the coating, the new MIT material repels even oily liquids with low surface tension, such as pentane.

“Pentane is probably the lowest energy liquid you can have at atmospheric pressure, and we were able to get drops of that just rolling around on our surface,” says Cohen.

It’s so robust that even when droplets of hexadecane – with a surface tension of 27.5 mN/m – are dropped onto the surface, they simply bounce and retain their spherical shape (see video, above).

Philippe Brunet at the Mechanics Laboratory of Lille, France, is impressed with the material.

“To my knowledge, no such universal repelling properties have been observed before this work,” he says. “What’s quite convincing is that the robustness was evidenced by drop impact experiments.”

Many concrete and glass companies have been interested in similar surfaces to improve their materials, he says. “But when you put this texture on the surface of a solid it is very easily destroyed – [the toadstools] are quite fragile.”

If they could be made more robust, they could make easy-to-clean surfaces that are difficult to soil with either watery or oily dirt.